As to a hydraulic drive system for driving a plurality of hydraulic actuators by a single variable displacement hydraulic pump, there is known a so-called load sensing control system in which a delivery rate of the hydraulic pump is controlled in such a manner as to supply only the flow rate demanded by the hydraulic actuators. The load sensing control system is described in, for example, West German Patent No. 3,321,483, JP, A, 60-11706 which are equivalent to U.S. Pat. No. 4,617,854) and JP, A, 2-261902.
The load sensing control system (hereinafter referred to as an LS control system) comprises a variable displacement hydraulic pump, a plurality of hydraulic actuators connected to the hydraulic pump in parallel, a plurality of flow control valves for respectively driving the plurality of hydraulic actuators, a plurality of control levers for instructing respective flow rates to the plurality of flow control valves, a circuit for detecting a maximum one of the load pressures of the plurality of hydraulic actuators, and a pump regulator for controlling a delivery rate of the hydraulic pump so that a delivery pressure of the hydraulic pump is held higher by a fixed value than the maximum load pressure.
When any one of the control levers is operated, the associated flow control valve is opened with an opening corresponding to an input amount from the control lever (i.e., a demanded flow rate), whereby a hydraulic fluid from the hydraulic pump is supplied to the associated hydraulic actuator through a pressure compensating valve and the flow control valve. Simultaneously, a load pressure of that hydraulic actuator is introduced as the maximum load pressure to the pump regulator which controls the pump delivery rate so that the pump delivery pressure is held higher by a fixed value than the maximum load pressure. At this time, when the input amount from the control lever (i.e., the demanded flow rate) is small, the opening of the flow control valve is also small and so is a flow rate of the hydraulic fluid passing through the flow control valve, so that the pump delivery pressure is held higher by a fixed value than the maximum load pressure at the small pump delivery rate. When the input amount from the control lever (i.e., the demanded flow rate) is increased, the opening of the flow control valve is also increased and so does the flow rate of the hydraulic fluid passing through the flow control valve, whereupon the pump delivery rate is increased to keep the pump delivery pressure higher a fixed value than the maximum load pressure.
Meanwhile, in the system making control of the pump delivery rate in that way, when plural hydraulic actuators are simultaneously driven by operating plural control levers, the flow control valve associated with the hydraulic actuator on the lower load side produces a larger differential pressure across the same than that on the higher load side, and the hydraulic fluid is supplied at a larger flow rate to the hydraulic actuator on the lower load side. The combined operation of those plural hydraulic actuators can no longer be performed in accordance with an opening ratio between the flow control valves (i.e., a demanded flow rate ratio). To prevent such a disadvantage, the LS control system includes a pressure compensating valve disposed upstream of the flow control valve for controlling a differential pressure across the flow control valve. When the differential pressure across the flow control valve associated with the hydraulic actuator on the lower load side becomes large during the combined operation, the upstream pressure compensating valve is operated in a valve-closing direction to restrict the flow rate, thereby reducing the differential pressure across that flow control valve. As a result, the differential pressures across the flow control valves on both the higher and lower load sides are maintained at substantially the same value, enabling the associated plural actuators to be simultaneously driven in accordance with the opening ratio between the flow control valves (i.e., the demanded flow rate ratio).
With the LS control system, as mentioned above, since the delivery rate of the hydraulic pump is controlled depending on the demanded flow rate, a part of the pump delivery rate which is wastefully consumed can be reduced to make economical operation possible. In order to surely perform the combined operation, the pressure compensating valve is required for controlling the differential pressure across the associated flow control valve.
Relating to the LS control system, particularly, there is also known U.S. Pat. No. 4,712,376 which discloses a system wherein the total of input amounts from all the control levers (i.e., demanded flow rates) is calculated for the purpose of controlling respective openings of the flow control valves. This disclosed system is intended to cope with a lack of the pump delivery rate during combined operation of driving plural actuators, by restricting the respective openings of the flow control valves depending on the amount of such a deficiency, so that the combined operation is performed in accordance with a demanded flow rate ratio. In addition, though not directly related to the LS control, JP, A, 52-76585 discloses a system in which a flow rate of the hydraulic fluid supplied to a hydraulic actuator is detected for controlling an opening of an associated flow control valve so that the flow rate is held in match with a demanded flow rate.